This invention relates to a method and apparatus for easily detecting the water-side flow instability in steam generators.
In steam generators for fast breeder reactors, gas furnaces and fossil fuel furnaces in general thermal power plants, there is a possibility that the flow instability will occur on the water side. The flow instability described above means the phenomenon in which fluid, i.e. water and vapor, pulsates or vibrates self-excitedly in heat tubes. Continued operation of the steam generator, overlooking the existance of such flow instability, may incur damages to the heat tube due to thermal fatigue, which in turn injures safety of the steam generator. Especially in the sodium-heated steam generator, the damage of the heat tube will cause sodium-water reaction which may lead to a serious disaster. For this reason, the development of the method and apparatus for easily and reliably detecting the occurrence of the flow instability has been strongly desired.
One of the conventional methods for detecting the flow instability is the one employing noise analysis. (T. Tamaori, J. Kubota et al. "Flow Instability Detection in Sodium-Heated Steam Generator by Noise Analysis", International Symposium on Nuclear Power Plant Control and Instrumentation, Cannes, France 24-28 April, 1978). This method is based on the noise analysis of process signals measured at the inlet and outlet of a steam generator, and is required to check the correlation between the process signals measured at the inlet and outlet to detect the flow instability. However, this method has the following disadvantages: the processing of signals is complicated owing to the computation of correlation; the equipment according to this method is expensive because it requires minicomputers or microcomputers; and there is a large time lag when the flow is unsteady. For the above reasons, this method cannot satisfactorily be applied to actual steam generators in operation. Other methods now being considered for detecting the flow instability are to insert a thermocouple into the heat tube to detect the fluctuation of the temperature or to directly install a feedwater flow meter to the heat tube. These methods may be effective for research or experimental equipments but almost inapplicable to actual steam generators in operation in the light of safety and reliability.